Device for generating a high voltage supply

ABSTRACT

A voltage generator comprises a capacitor and positive and negative power supply rails. Normally non-conducting first and second field-effect transistors are arranged to isolate the capacitor from the power supply rails in the non-conducting state thereof and to connect the power supply rails to respective plates of the capacitor in a conducting state thereof. Means are provided to trigger the first and second transistors into the conducting state for a limited period to charge the capacitor and then isolate the capacitor when the transistors revert to their normal state. A normally non-conducting field-effect transistor is operative, on isolation of the capacitor subsequent to charging thereof, to adopt a conducting state in which such third transistor connects one of the plates of the capacitor to the opposite supply rail to that to which it can be connected by the respective first or second transistor, whereby to produce an enhanced voltage on the other capacitor plate. The enhanced output is delivered across an output capacitor connected via a fourth field-effect transistor to the said other plate of the first mentioned capacitor.

United States Patent [191 Fry [45] Feb. 5, 1974 DEVICE FOR GENERATING AHIGH VOLTAGE SUPPLY [57] ABSTRACT [75] Inventor: Peter William Fry, D ht A voltage generator comprises a capacitor and posi- England tive andnegative power supply rails. Normally nonconductin first and secondfield-effect transistors are [73 Asslgnee: Integrated Phommamx L'mitedarranged t o isolate the capacitor from the power sup- [22] Filed: July13, 1972 ply rails in the non-conducting state thereof and to connectthe power su pl rails to respective plates of [21] Appl 27l239 thecapacitor in a con uciing state thereof. Means are provided to triggerthe first and second transistors into [30] Foreign Application PriorityData the conducting state for a limited period to charge the July 26,1971 Great Britain 34,982/71 capacit" and isOlate the capacitor when thetransistors revert to their normal state. A normally 52 US. Cl 307/110,307/246, 321/15 "On-conducting field-fed transistor is Operative,

51 I111. Cl. 1102111 3/18, H03k 17/66 isolation of the capacitorSubsequent to Fharging [58] Field 6: Search. 307/109, 110, 246; 320/1; e2 adopt a conducting State In which Such 323/93; 321/15 third transrstorconnects one of the plates of the capacitor to the opposite supply railto that to which it [56] References Cited can be connected by therespective first or second UNITED STATES PATENTS transistor, whereby toproduce an enhanced voltage on the other capacitor plate. The enhancedoutput is delivered across an output capacitor connected via a INPUT 72PULQE UPPLY 7 7 fourth field-effect transistor to the said other plateof the first mentioned capacitor.

7 Claims, 1 Drawing Figure DEVICE FOR GENERATING A HIGH VOLTAGE SUPPLYBACKGROUND OF THE INVENTION This invention relates to voltagegenerators.

.A subsidiary supply voltage is often required for an M.I.S.(Metal-insulator-semiconductor) integrated circuit, which voltage is ofgreater magnitude than that produced by the main power supply but needonly supply a very low or negligible current. Such voltage, may, forexample, be for connection to the gates of M.I.S.

OBJECTS AND SUMMARY OF THE INVENTION An object of the present inventionis to provide a voltage generator which may, in practice, be designed asa part of the associated integrated circuit which it is to supply withvoltage and which may avoid some or all of the above mentioneddisadvantages.

Broadly, the invention provides a voltage generator including acapacitor and associated switching circuitry which is operable initiallyto charge the capacitor to a first potential and then effectively toisolate the capacitor and apply a reverse potential to one of the platesthereof, thereby to produce an enhanced potential on the other plate. I

More specifically, the invention provides a voltage generator includingfirst and second electrically controllable switching devices arranged tobe operable to connect opposite plates of a capacitor to the positiveand negative rails of a power supply respectively, and

a third electrically controllable switching device arranged to beoperable to connect one capacitor plate to the opposite rail to that towhich it can be connected by the respective one of the first or secondswitching devices, the arrangement being such that, in operation of thegenerator, initially the third switching device is open and the firstand second switching devices close to charge the capacitor from thepower supply, then the first and second switching devices open toisolate the capacitor from the power supply rails and the thirdswitching device closes to apply a reverse potential to the onecapacitor plate thereby to produce on the other capacitor plate anenhanced voltage which is fed to an output. 7

In an embodiment of the invention, closing of the first and secondswitching devices to charge the capacitor is controlled by applicationof a pulse to a control electrode of one of the said first and secondswitching devices.

Suitably, the one of said first and second switching devices connectedto the said other capacitor plate is arranged to open in response tosaid other capacitor plate attaining a predetermined potential.

Advantageously, the said other capacitor plate may be connected tooutput capacitor via fourth electrically controllable switching devicearranged to feed the enhanced voltage to the output capacitor acrosswhich the enhanced voltage output may be taken.

2 Preferably, the arrangement is such that the third switching devicecloses in response to said other capacitor plate attaining apredetermined potential.

In practice, each of the switches may be constituted by an M.I.S.T.

In one form of generator embodying the invention, the third switchingdevice comprises an M.I.S. transistor having its gate electrodeconnected to said other capacitor plate.

Preferably, the gate electrode of the third switching device is alsoconnected to one plate of a protective capacitor having its other plateearthed.

Desirably, the first and second switching devices are operated toconnect the first mentioned capacitor to the positive and negative railsrepeatedly, thereby to enhance further the voltage attained by the saidother capacitor plate.

In order that the invention may be readily understood, an embodimentthereof will now be described in more detail, byway of example, withreference to the accompanying drawing which is a circuit diagram of oneembodiment of voltage generator according to the invention.

DETAILED DESCRIPTION In the embodiment shown in the drawing, a voltagegenerator is formed as part of an M.I.S. integrated circuit. A capacitor1 of the generator has one plate 2 connected to respective currentcarrying electrodes 3 and 4 of two serially connected normallynonconducting M.I.S. transistors 5 and 6. The transistors 5 and 6 havetheir respective other current carrying electrodes 7 and 8 connected topositive and negative supply rails 9 and 10 to each of which therespective transistor connects the plate 2 when biased into a conductivestate.

The gate electrode 11 of transistor 5 is connected to an input line 12,to which input pulses are supplied to bias transistor 5 into itsconductive state in which it connects capacitor plate 2 to the positivesupply rail 9. The gate electrode 13 oftransistor 6 is connected to theother plate 14 of the capacitor 1, so that such transistor is biasedinto its conductive state, in which it connects capacitor plate 2 to thenegative supply rail'l0, when the voltage on gate electrode 13 reaches apredetermined value. A protective capacitor 15 of small capacitance isconnected between the gate electrode 13 of transistor 6 and earth.

The capacitor plate 14 of capacitor 1 is also connected to currentcarrying electrode 16 of a normally non-conducting M.I.S. transistor 17having its other current carrying electrode 18 and its gate electrode 19connected to negative supply rail 10. The transistor 17 becomes biasedinto a conductive state, in which it connects capacitor plate 14 to thenegative supply rail 10 when the voltage 'on such capacitor plateattains a predetermined value.

The gate electrode 20 and one current carrying terminal 21 of a furtherM.I.S. transistor 22 is also connected to the capacitor plate 14 to feedan enhanced voltage produced on such plate during operation of thegenerator via current carrying terminal 23 to one plate of a capacitor24 having its other plate connected to the negative supply rail 10. Anoutput voltage V is taken across the capacitor 24 on outputrail 25.

In operation of the generator, an input pulse on line 12 is fed to thegate electrode 11 and biases transistor 5 into its conductive state.This causes the potential V of the plate 2 of capacitor 1 to rise towithin about 1 volt of the voltage V, of positive rail 9. The potentialV of the other plate 14 of capacitor 1 is pulled up with potential V bythe action of the capacitor until the threshold voltage V of thetransistor 17 is reached, when transistor 17 conducts and charges thecapacitor 1 to the negative rail voltage V less about 1 volt plus thethreshold voltage V Prior to the conduction of transistor 17, thevoltage V of output rail 25 is limited to a value equal to the sum ofthe threshold voltages of transistors 17 and 22 above the negative railvoltage V When the biasing potential caused by the input pulse isremoved from transistor 5, such transistor reverts to a non-conductingstate whilst transistor 6 is switched to its conductive state by thepotential on the plate 14 of capacitor 1. Thus, the potential V ofcapacitor plate 2 falls as a result of the current flowing in transistor6 and, as it does so, the potential V of the other capacitor plate 14also falls due to the capacitive coupling of the plates 2 and 14. Thecapacitor 1 does not lose any charge since transistor 17 is biased intoa nonconducting state as soon as V,, falls further in value. Therefore,transistor 6 is kept in its conductive state until potential Vsubstantially attains the negative rail voltage V when potential V,, hasfallen by an amount equal to:

where AV is the fall in V C is the capacitance of capacitor 1, and C isthe capacitance of capacitor 15. The final potential V on capacitorplate 14 is thus:

where V is the threshold voltage of transistor 17 and is much less thanV Capacitor 24 is charged by this potential via transistor 22 and, ifpulses are repeatedly appliedto transistor in this fashion, the outputvoltage V on output rail 25 attains the value of:

where it has been assumed that the threshold voltage of transistor 22 isalso V C, is very much greater than C so that the quantity C,/(C, C isalmost equal to unity and, to an approximation,

V may be written as:

V 2V AV which clearly can be considerably more negative than the valueof V itself.

I claim:

1. A voltage generator including:

a capacitor;

' positive and negative power supply rails;

first and second normally non-conducting M.l.S. transistors each ofwhich is connected between a respective plate of the capacitor and arespective power supply rail; means to apply electrical pulses to a gateelectrode of one of the first and second transistors to render the firstand second transistors conducting for a limited period to charge thecapacitor and then isolate the capacitor on the first and secondswitching devices reverting to their non-conducting state; a normallynon-conducting third M.l.S. transistor connected between one of theplates of the capaci- 5 tor and the opposite power supply rail to thatto which the one capacitor plate is connectable by the respective one ofthe first and second transistors; and,

output means connected to the other capacitor plate;

the third transistor having its gate electrode connected to the saidother capacitor plate for biasing the third transistor to a conductingstate in response to the said other capacitor plate attaining apredetermined voltage, whereby the third transistor is operative, onisolation of the capacitor subsequent to charging thereof, to connectthe one capacitor plate to the said opposite power supply rail toproduce an enhanced voltage on the said other capacitor plate.

2. A voltage generator according to claim 1, wherein the output meanscomprises a fourth M.l.S. transistor and a second capacitor, the fourthtransistor having its gate electrode and one current carrying terminalconnected to the said other capacitor plate, the second capacitor beingconnected between the other current carrying terminal of the fourthtransistor and the power supply rail to which the said one capacitorplate is connected by the third transistor.

3. A voltage generator including:

a capacitor having first and second plates;

positive and negative power supply rails;

normally open first and second electrically controllable switchingdevices each of which is connected between a respective one of saidfirst and second plates of the capacitor and a respective power supplyrail;

means to trigger the first and second switching devices into the closedstate thereof for a limited period to charge the capacitor and to thenisolate the capacitor on the first and second switching devicesreverting to their normally open state;

a normally open third electrically controllable switching deviceconnected between one of the first and second plates of the capacitorand the power supply rail to opposite that which the said one capacitorplates is connectable by the respective one of the first and secondswitching devices;

and, output means connected to the other capacitor plate; the thirdswitching device being operatively connected to the said other capacitorplate for triggering to a closed state in response to the said othercapacitor plate attaining a predetermined voltage, whereby the thirdswitching device is operative, on isolation of the capacitor subsequentto charging thereof, to connect the said one of the first and secondcapacitor plates to the said opposite power supply rail to produce anenhanced voltage on the said other of the first and second capacitorplates.

4. A voltage generator according to claim 3, wherein the third switchingdevice comprises an M.l.S. transistor having its gate electrodeconnected to the said other of the first and second capacitor plate.

5. A voltage generator according to claim 4, wherein the gate electrodeof the third switching device is connected to one plate of a protectivecapacitor having its other plate connected to ground.

6 trollable switching device connected to the said other capacitorplate, and a second capacitor, the fourth switching device beingarranged to feed the enhanced voltage to the second capacitor acrosswhich a voltage output can be taken.

1. A voltage generator including: a capacitor; positive and negative power supply rails; first and second normally non-conducting M.I.S. transistors each of which is connected between a respective plate of the capacitor and a respective power supply rail; means to apply electrical pulses to a gate electrode of one of the first and second transistors to render the first and second transistors conducting for a limited period to charge the capacitor and then isolate the capacitor on the first and second switching devices reverting to their non-conducting state; a normally non-conducting third M.I.S. transistor connected between one of the plates of the capacitor and the opposite power supply rail to that to which the one capacitor plate is connectable by the respective one of the first and second transistors; and, output means connected to the other capacitor plate; the third transistor having its gate electrode connected to the said other capacitor plate for biasing the third transistor to a conducting state in response to the said other cApacitor plate attaining a predetermined voltage, whereby the third transistor is operative, on isolation of the capacitor subsequent to charging thereof, to connect the one capacitor plate to the said opposite power supply rail to produce an enhanced voltage on the said other capacitor plate.
 2. A voltage generator according to claim 1, wherein the output means comprises a fourth M.I.S. transistor and a second capacitor, the fourth transistor having its gate electrode and one current carrying terminal connected to the said other capacitor plate, the second capacitor being connected between the other current carrying terminal of the fourth transistor and the power supply rail to which the said one capacitor plate is connected by the third transistor.
 3. A voltage generator including: a capacitor having first and second plates; positive and negative power supply rails; normally open first and second electrically controllable switching devices each of which is connected between a respective one of said first and second plates of the capacitor and a respective power supply rail; means to trigger the first and second switching devices into the closed state thereof for a limited period to charge the capacitor and to then isolate the capacitor on the first and second switching devices reverting to their normally open state; a normally open third electrically controllable switching device connected between one of the first and second plates of the capacitor and the power supply rail to opposite that which the said one capacitor plates is connectable by the respective one of the first and second switching devices; and, output means connected to the other capacitor plate; the third switching device being operatively connected to the said other capacitor plate for triggering to a closed state in response to the said other capacitor plate attaining a predetermined voltage, whereby the third switching device is operative, on isolation of the capacitor subsequent to charging thereof, to connect the said one of the first and second capacitor plates to the said opposite power supply rail to produce an enhanced voltage on the said other of the first and second capacitor plates.
 4. A voltage generator according to claim 3, wherein the third switching device comprises an M.I.S. transistor having its gate electrode connected to the said other of the first and second capacitor plate.
 5. A voltage generator according to claim 4, wherein the gate electrode of the third switching device is connected to one plate of a protective capacitor having its other plate connected to ground.
 6. A voltage generator according to claim 3, wherein the means to trigger the first and second switching devices comprises means to apply electrical pulses to a control electrode of one of the first and second switching devices.
 7. A voltage generator according to claim 3, wherein the output means comprises a fourth electrically controllable switching device connected to the said other capacitor plate, and a second capacitor, the fourth switching device being arranged to feed the enhanced voltage to the second capacitor across which a voltage output can be taken. 